The present invention relates to a photoresist article having enhanced image contrast.
It is well known in the art to produce conventional photoresist formulations such as those described in U.S. Pat. Nos. 3,666,473, 4,115,128 and 4,173,470, which are incorporated herein by reference. These include alkali-soluble phenol-formaldehyde novolak resins together with light-sensitive materials, usually a substituted naphthoquinone diazide compound. The resins and sensitizers are dissolved in an organic solvent or mixture of solvents and are applied as a thin film or coating to a substrate suitable for the particular application desired.
The ability of a resist to reproduce very small dimensions, on the order of a micron or less, is extremely important in the production of large scale integrated circuits on silicon chips and similar components. Circuit density on such a chip can only be increased, assuming the same photolithographic techniques are employed, by increasing the resolution capabilities of the resist. O-quinone diazide sensitized phenol-formaldehyde resists have high sensitivity and submicron resolution when the resist layer thickness is sufficiently small that diffraction and absorption effects do not limit resolution. Thick resist layers of this type (greater than one micron) have a high aspect ratio and a much reduced resolution due to optical diffraction and absorption effects.
When pattern exposure of a photoresist, such as a o-quinone diazide sensitized phenol-formaldehyde resist, is done optically, the high numerical aperture lenses have limited use because of their narrow depth of focus, necessitating the use of thin films. It is practical to use an electron beam but resolution is limited by scattering effects. Exposure by contact printing also avoids the depth of focus problem but it involves other disadvantages. Contact printing tends to scratch masks, so that contact masks have a short life and should be more durable than masks fabricated for projection printing. Proximity printing rather than contact printing extends the life of the mask but diffraction effects are worse. Diffraction effects may be reduced somewhat by reducing the wavelength of the exposure light but this type of improvement is ultimately limited by the light sensitivity and absorption characteristics of the resist. Because of these problems it has not been practical generally to fabricate sufficiently high resolution thick masks from diazo sensitized phenol-formaldehyde resins.
In order to reduce the wavelength of exposure light a class of deep u.v. sensitive resists is known. Such may be composed of radiation degradable alkyl acrylate or methacrylate polymers.
Such resists and their use are described for example in U.S. Pat. Nos. 3,538,137; 3,934,057 and 3,984,582 which are hereby incorporated by reference. Alkyl methacrylate polymers such as polymethyl methacrylate and copolymers are typically patternwise exposed by u.v. radiation or electrons. These resin systems require large exposure doses, therefore imaging time is very large and throughput low.
Alkyl methacrylate polymers are degradable with other high energy radiation as well. X-ray radiation in the 5 angstrom to 50 angstrom range produces a particularly sharp edge and a very high aspect ratio is possible with this type of radiation. However, computer controlled direct writing with x-ray radiation is not practical with current technology and projection printing with x-ray radiation is not technically feasible. Contact printing and proximity printing can be done with x-ray radiation but good materials for fabricating suitable masks are not available.
Alkyl methacrylate polymer resist layers may also be selectively degraded by patterned exposure to deep ultraviolet light at a wavelength of less than 3000 angstroms. Direct writing is again not practical while projection exposure limits resolution. Contact exposure masks may be formed but such layers are difficult to form uniformly.
It is also known that contrast may be enhanced by forming the so-called portable conformable masks or contrast enhancement layer photoresists. By this method, a positive working photoresist layer is applied to a substrate and on this is a second photosensitive layer is imaged to create a photomask which is intimately adhered to the lower photoresist for a second exposure. This close mask contact reduces diffraction thus increasing contrast.
Generally any two photosensitive layers, one spun on top of the other, will cause intermixing between the layers, in particular when both resists are organic solvent based. In order to avoid this intermixing a barrier layer is introduced between the two photosensitive layers, such that the solvent used to spin on the barrier layer does not dissolve the bottom resist and the solvent for the top layer does not dissolve the barrier polymer.